Method of purifying unsaturated hydrocarbons by extractive distillation with side stream removal and solvent mix

ABSTRACT

IMPROVED CONTROL AND GREATER CAPACITY OF AN EXTRACTIVE DISTILLATION TOWER CAN BE OBTAINED BY RETURNING THE REFLUX TO THE TOWER WIGHDRAWING ALL OR A PORTION OF THE EXTERNAL REFLUX FROM THE TOWER AND PREMIXING F THE SOLVENT FEED AND EXTERNAL REFLUX PRIOR TO INTRODUCING THE SOLVENT TO THE TOWER. BY PROCEEDING IN THIS MANNER BETTER SOLUBILIZATION OF THE EXTERNAL REFLUX IN THE SOLVENT FEED IS OBTAINED THUS INCREASING THE AMOUNT OF REFLUX THAT CAN BE HANDLED BY THE SOLVENT WITH FLOODING. THE EXTERNAL PREMIXING ALSO ALLOWS ADJUSTMENT OF THE SOLVENT FEED TEMPERATURE TO COMPENSATE FOR THE VARIATIONS IN ENDOTHERMIC HEAT OF SOLUBLITY FROM VARIATIONS IN THE HYDROCARBON COMPOSITION IN THE TOWER.

' Aug. 1, 1972 SOLVENT STRlPF'ER EXTRACTIVE DISTILLATION T. o. FUNKHOUSER 2 METHOD OF PURIFYING UNSATURATED HYDROCARBONS' BY EXTRACTIVE DISTILLATION WITH SIDE STREAM REMOVAL AND SOLVENT MIX Filed June 1, 1970 TERRY D. FUNKHOUSER INVENTOR.

ATTORNEY United States Patent U.S. Cl. zos- -ss 8 Claims ABSTRACT OF THE DISCLOSURE improved control and greater capacity of an extractive distillation tower can be'obtained by returning thereflux to the tower withdrawing all or a portion of the external refluxfrom the tower and premixing of the solvent feed and external reflux prior to introducing the solvent to the tower. By proceeding in this manner better solubilization of the external refluxin the solvent'feed is obtained thus increasing the amount of reflux that can be handled by the solvent without flooding. The external premixing also allows adjustment of the solvent feed temperature to compensate for the variations in endothermic heat of solubility from'variat'ions, in-the hydrocarbon composition in the tower.

Thisinvention relates to an improvement in the separation of hydrocarbons of 4 to 5 carbon atoms, including. unsaturated hydrocarbons, using extractive distillation.

Unsaturated hydrocarbons of 4 to 5 carbon atoms have been prepared commercially by the dehydrogenation of more saturated hydrocarbons such as alkanes, or alkenes. These processes provide a mixture of hydrocarbons containing varying degrees of unsaturation, for example, butene or butane or mixtures thereof may be dehydrogenatedto produce a mixture of hydrocarbons containing butane,-butene, butadiene and the minor amount of alkynes such as vinylacetylene and methylacetylene. Similar mixtures of hydrocarbon products may be obtained from other reactions such. as the catalytic or non catalytic cracking of hydrocarbons. Generally, clue to the complexity and similarity of the boiling points of the components, the separation of these mixtures cannot be economically performed by fractional distillation alone. It has been conventional to employ extractive distillation to separate hydrocarbons having very similar or narrowly separated boiling point or hydrocarbons forming azeotropes during distillation. Examples of extractive distillation processes may be found in US. Pats. 3,000,794, 3,026,253 and 3,317,627. L Extractive distillation has been an invaluable asset in obtaining a high degree of purity of the various desired hydrocarbon products.'However, the'operation of an extractive distillation on a commercial scale can be a very difficult operation becauseof the ease with which the delicate balance of solvent, feed, temperature and the like may be upset.

In operating a solvent tower the fractionating capabilities of the tower are related directly to the internal reflux or dissolved hydrocarbon flowing down through the tower with the solvent. For maximum efliciency of the distillation the amount of dissolved hydrocarbon should be as close as possible to the solubility limit. When the solubility limit is exceeded the towers frequently go into uncontrollable stacks or flooded conditions. These upsets can result in considerable loss of specification product and costly reruns. The prior practice has been to control the amount of internal reflux by controlling the solvent feed temperatures as well as controlling the amount of external reflux. In practice the solvent is normally'fed into the tower a few trays from the top in order to provide for removal of solvent vapors from the overhead product. The result of this conventional ar-- rangement was that the hydrocarbon reflux from the top of the tower plus the induced reflux, that is, the condensation of hydrocarbon vapors at the point of solvent entry all had to be dissolved in the solvent on the solvent feed tray. The failure of proper solution of the hydrocarbons into the solvent is now considered to be a primary factor in tower flooding.

It is an object of the present invention to provide a process whereby flooded conditions will be avoided. Another object of this invention is to provide a process that is more controllable than previously. Yet another object is to provide a process that will allow much smoother tower operation. A further object of the invention is to provide more positive control of an extractive solvent tower. Another object of the present invention is to provide a method of controlling the temperature of the total feed to the solvent feed tray. A particular object of the invention is to provide for dissolving all of the external reflux in the solvent. These and other objects will become apparent from the following discussion.

The drawing is a schematic representation of one method of obtaining the objects of the present invention. An organic feed comprising a hydrocarbon mixture containing C or C alkadiene, alkene, and alkane is fed to an extractive distillation tower A where it is contacted with a solvent. The solvent is added to the extractive distillation tower A by being premixed with a draw stream 20 from the upper portion of A. The solvent is preferential for the most unsaturated compound. A rich solvent is fed to solvent stripper B where the contained C and C are removed as overhead. The lean solvent from B then recycles to A.

It has now been found that the objects of the present invention can be obtained by mixing a portion of the external hydrocarbon reflux which has been withdrawn from the extractive distillation tower with the solvent prior to adding the solvent to the extractive distillation tower. By premixing in this fashion it is possible to obtain essentially complete solution of that portion of the external hydrocarbon reflux in the solvent. It may be 'desirable undersome operations to premix all or substantially all of the external hydrocarbon reflux with the incoming solvent. In any event, by dissolving a portion of the external hydrocarbon reflux from the tower in the solvent prior to adding the solvent to the extractive distillation tower, the solution within the tower of the external hydrocarbon reflux can be controlled at any level desired. Generally at least 50% and preferably at least of the external reflux is withdrawn and premixed with the incoming solvent feed. The induced reflux is still present but under the process described its effects are minimized or eliminated so as not to upset the internal reflux, i.e., the dissolved hydrocarbon flowing through the tower with the solvent. An additional advantage has been found with the present process in that the external premixing of solvent and external reflux provides a higher degree of temperature control of the total feed to the solvent tray. In prior methods of operation changes in the hydrocarbon composition resulted in varying solvent feed tray temperatures because of the endothermic heat of solution. By premixing the external hydrocarbon reflux and solvent the temperature variation due to changes in the hydrocarbon composition can be compensated by heating or cooling as necessary to maintain a substantially constant temperature on the solvent feed tray In order that those of skill in the art may more fully appreciate the nature and method of the invention, it will T 3 be-morespecifically described inconnection withthe accompanying drawing which isa flow sheet of one mode of the invention. The process will be illustrated by the use of particular pieces of equipment but it is understood that a single piece of equipment may be separated into several pieces of equipment or conversely several pieces of equipment may be combined so long as the same result or effect is obtained therefrom. Conventional auxiliary equipment such as pumps, heating and cooling means, compressors, reboilers andthe like have not been shown as this type of equipment and use in such systems is well known to those in the art. U s

The vaporized feed 15 to the extractive distillation tower A comprises unsaturated hydrocarbons of 4 to 5 carbon atoms such as alkadienes contaminated with minor amounts of alkyne of the same number of carbon atoms; a mixture of alkenes and al'kadienes, alkenes, alkenes and alkadienes; or alkenes, alkadienes and alkynes. For example, the alkadiene may be butadiene-1,3, contaminated with vinyl acetylene or may be isoprene contaminated with isopropyl or isopropenyl acetylene and possibly l-pentyne, or 2-pentyne. In a similar fashion the feed may be butene-lcontaminated with aminor amount of butadiene-l,3. Normally the feed stream will be a hydrocarbon stream as described and may contain in addition thereto other hydrocarbons such as those having 2 to 8 carbon atoms. The extractive distillation column A may be any conventional equipment suchas a plate or packed type column. The bottoms 11 contain solvent and unsaturated hydrocarbon. In solvent stripper B the unsaturated hydrocarbon is overhead 4. The overhead 4 is condensed at 6 and accumulated at 2. A portion of the condensed overhead is recycle 8. The remainder is taken off as unsaturated product which can be further purified if desired by a repetition of similar extractive distillations, straight fractionation, or any other known purification method. The bottoms 12 in the solvent stripper B will contain principally solvent, although there may be some retained unsaturated material. The lean solvent 12 is recycled for use in the extractive distillation tower A, and is injected into stream which is a portion of the external recycle hydrocarbon which was withdrawn from tower A. Stream 20 will be a more saturated hydrocarbon than found in recycle stream 8 for example. Stream 20 is withdrawn at a point above the solvent feed, generally from the tray above the solvent feed tray, although it may be a combination of several of the upper trays. The two streams, 12 and 20 pass through an inline mixer 16 and then through a cooler 18. Combined streams 12 and 20 are then fed to the tower A on the solvent feed tray.

The overhead 3 from tower A goes through a condenser 5 and accumulator 14 from whence a portion 7 is recycled to the tower and stream 9 is withdrawn.

In some embodiment water may be present as a part of the solvent system. In such instances water in the overheads of both towers A and B is knocked out respectively as streams 17 and 13. These combined streams can be added to lean solvent 12 as makeup or water from other sources (not shown) can be used entirely. Solvent makeup is made through stream 19 as needed.

The solvents employed in the extractive distillation tower may include any of the polar solvents known to separate relatively more unsaturated hydrocarbons from less unsaturated hydrocarbons such as acetone, acetonitrile, dimethyl formamide, dimethyl sulphoxide, furfural, n-methyl pyrrolidone, methylethyl ketone, dimethyl acetamide, 3-methoxypropionitrile, mixtures of these solvents froma catalytic dehydrogenation andcontains-OA. VOL,

and with waterLGenerally up to about 25 weight percent percent propane, propylene, 0.3 vol. percent isobutane, 55.0 vol. percent n-butane, 12.4 vol. percent butene-l, 0.9 vol. percent isobutene, 15.9 vol. percent butene-ZL, 8.6 vol. percent butane-2H, 5.9 vol. percent butadiene, and 0.6 vol. percent pentane, pcntene. Hydrocarbonfeed, 15 is fed to the extractive distillation tower A at a rate of about 239 barrels per hoiii'rThe bottoms 11 consists of 88.0 vol. percent furfural, 7.6 vol. percent water, 4.4 vol.

percent C hydrocarbons and pass to the'solvent stripper B at the rate of about 2080barrels per hour. Aproduct stream 10 is taken off the condensedhydrocarbon overhead. at the rate of about barrels per hour. Stream-10;

contains 1.5 vol. percent butane, 19.6 vol. percent but ene l, 1.4 vol. percent isobuten'e,*39'.4 vol. percent butene-ZL, 22.2 vol. percent butene-ZH, 15.2 vol. percent butadiene, and 0.7 vol. percent pentenes. This stream can be further purified for example fractionated to separate butadiene; 1,3 from butene-Z to provide a 98% pure butadiene-1,3.f The rich solvent stream '11 enters the; solvent "stripper B A results in a more stable operation 'of tower A which will in the present process accommodate a higher daily throughput because of the elimination of the solubility problems of external reflux on the solvent feed tray.

The lean solvent 12 can have makeup solvent and/or water added as necessary. Since the lean solvent 12 is coming out of the stripper B at about 320 F. it is usually cooled (not shown) by indirect heat exchange in reboilers on tower A and in feed Vaporizers in which the incoming feed is vaporized and fed to tower A at about 150 F. Additional cooling may be used to bring the'temperature of the lean solvent 12 down to about 140 F. at thej point it enters stream 20. The lean solvent 12 is being recycled to tower A at the rate "of about 1990 barrels per hour. Stream 20 is the external recycle of the condensed hydro carbon overhead 3 of tower A. Stream 20 is taken off of the tray above the solvent feed'tray at the rate of about 166 barrels per hour and is composed of 0.7 vol. percent propane, propylene, 0.5 vol. percent isobutane, 88.8 vol. percent n-butane, 7.8 vol. percent butene-l, 0.5 vol. percent isobutene, 1.1 volpercent butene-Z' and 0.6 vol. percent pentanes. This is actually over 100% of the condensed hydrocarbon refiux 7 which is returned to tower A at the rate of 160 barrels per hour, and is the result of the condensation of vaporous material in the tower, (i.e., 118 F.) condensed hydrocarbon reflux. A portion. 9 of the condensed hydrocarbon overhead iswithdrawn and returned to serve as a source of feed-for the dehydrogenation from which'the feed 15 isobtained. The extractive distillation tower A is operated at an overhead temperature of about 120 to 135 F., thus stream 20 is at about F. when it contacts lean solvent stream 12'and'is mixed in 16. The endothermic heat of solution of the hydrocarbons in the furfural solvent further cools the combined streams and a cooler 18 is used to bring the temperature down to about F. for entry into the tower A. Midpoint and bottoms temperatures fortower A will range from about to and 250 to290 re spectively.

By theadditionof a small amount of equipment, e.g., a

. pump, inline mixer, cooler and piping, to provide for the premixing as described "it is possible to operate aparticular extractive distillation operation at close to the capacity of equipment without costly upsets. By comparison it is found that the same extractive distillation as described herein it carried out by injecting the lean solvent feed 12 directly to tower A can handle a throughput of only about 91% of that possible according to the present invention. Without premixing as described, it is impossible to operate the equipment, even with modification of the process conditions, at the same high rate because of flooding.

The invention claimed is:

1. In the process of separating mixtures of C to C hydrocarbons of like carbon number and different degrees of saturation by extractively distilling the hydrocarbon mixture with a selective solvent for the more urisaturated hydrocarbon in an extractive distillation column, including the steps of introducing said selective solvent to said extractive distillation column, introducing said hydrocarbon mixture to said extractive distillation column at a point thereof below the point of introduction of said selective solvent, selectively extracting unsaturated hydrocarbon to form a liquid solvent fraction rich in the more unsaturated hydrocarbon, withdrawing a vaporous hydrocarbon fraction as overhead from the top of said extractive distillation column, condensing said vaporous hydrocarbon fraction, returning a portion of said condensed vaporous hydrocarbon fraction, as reflux, to said extractive distillation column, and withdrawing the solvent rich in the more unsaturated hydrocarbon from the bottom of said distillation column, wherein the improvement comprises (1) returning all said reflux to said extractive distillation column, 2) withdrawing a sidestream from said extractive distillation column equivalent in amount to at least 50 percent by volume of said reflux, said sidestream being withdrawn at a point on said extractive distillation column below the introduction of reiflux thereto and above the introduction of selective solvent thereto, and (3) mixing said sidestream with the selective solvent prior to introducing the selective solvent into said extractive distillation column.

2. The process according to claim 1 wherein the selective solvent is continuously introduced to said extractive distillation column and continuously withdrawing the solvent fraction rich in the more unsaturated hydrocarbon.

3. The process according to claim 2 wherein said sidestream added to the selective solvent is substantially equivalent in amount to said reflux returned to said extractive distillation column.

4. The process according to claim 1 wherein said sidestream added to the selective solvent is equivalent in amount to at least 85 percent of said reflux to said extractive distillation column.

5. The process according to claim 1 wherein the selective solvent consists of furfural and up to 25 weight percent of water.

6. The process according to claim 5 wherein the hydrocarbons to be separated consists of n-butane, n-butene and butadiene.

7. The process according to claim 1 wherein the liquid solvent fraction rich in the more unsaturated hydrocarbon is fed to a solvent stripping column, the selective solvent is withdrawn from said solvent stripping column as bottoms and recycled to said extractive distillation colum and a vaporous hydrocarbon fraction is obtained as overhead from said solvent stripping column.

8. The process according to claim 7 wherein the vaporous hydrocarbon fraction from the stripping column contains a major amount of butadiene and butenes and a minor amount of n-butane.

References Cited UNITED STATES PATENTS 2,434,796 1/1948 Hachmuth 203-54 3,008,880 11/1961 Dodge et a1. 203- 3,235,474 2/ 1966 Clay 203-64 3,496,069 2/ 1970 TIschopp et a1. 203-54 3,496,070 2/1970 Woerner et a1. 203--54 WILBUR L. BASCOMB, 111., Primary Examiner lU.S. Cl. X-R- 20362, 57, '58, 60, 54, 98, 99, DIG 19; 260--68l.5, 676 R "H050 Um'rEn STATES PATENT OFFICE I W CERTIFICATE OF CORRECTION Patent No. 8 ,202 Dated August 1, 9 Inventor) erry D. Funkhouser It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

H C 01. 3, line s-6:7 and 68, after "3-methoxypropionitrile, 'hreads "mixt of these solvents and with water"but should read and m1xtures of the solvents with water Signed and sealed this 14th day of Ja hua ry' 1975.

'(SEAL) 'Attestr cCOY 1:4. GIBSON JR. c. MARSHALL DANN I I AttestxngOfficer Commissioner of Patents 

